Stop deleting pusher_id on versions when the parent user is deleted

[JackMcDataGrail]

Summary

This PR deletes the dependent: :nullify option that, when a User was deleted, was setting the pusher_id of all their Version objects to nil.

Risks

This approach does not add any risk (since the IDs are opaque), but it does create the somewhat strange condition that there will be Versions pointing at non-existent users. Rails handles this for optional relationships by simply returning nil as if there isn't an ID at all, which is fine. The alternative approach would be to switch to soft-deletion for users. However, this could present its own set of problems due to the retention of "private" data past the user's explicit request to delete it.

For now, I propose we stop nulling out the ID, and leave soft-deletion to another PR.

[codecov]

Codecov Report

Merging #3766 (6b037f0) into master (77e3421) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3766   +/-   ##
=======================================
  Coverage   98.80%   98.80%           
=======================================
  Files         212      212           
  Lines        5198     5199    +1     
=======================================
+ Hits         5136     5137    +1     
  Misses         62       62           

Impacted Files	Coverage Δ
app/models/user.rb	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[simi]

Hello!

I was thinking about this for some time and I don't think this is good idea. It will leave database in inconsistent state with broken relations. Soft deletion could be the way to keep at least something about original pusher when user got deleted.

[segiddins]

@simi I think this is a step in the right direction, we can implement soft deletion separately

[simi]

ℹ️ soft deletion happening at #4376 🙏